1. Field of the Invention
The field of art to which this improvement pertains includes differential pressure regulator valve means for use in fluid operated control systems. More particularly, the valve means of this invention is constructed to control and regulate differential fluid pressures and it may be disposed, for example, in a control system for a hydrostatic transmission.
2. Description of the Prior Art
Briefly, this invention constitutes an improvement in pressure regulating control valve mechanisms. Such control valve mechanisms are often used in the prior art for regulating pressure to a fluid operated clutch of the type often found in the constant-mesh power-shift transmissions of industrial lift trucks and other machines. Specifically, such prior art control valve means are frequently constructed to permit an immediate relief of a portion of the clutch application pressure followed by a regulated decrease or increase in the fluid pressure supplied to the fluid clutch whereby the latter may be gradually disengaged or engaged by the operator. One particular use of this type of regulating valve, contemplates the control of a fluid-pressure actuated clutch for the transmission of an industrial lift truck where it is often necessary to hold the truck stationary while the lift fork is operated at a speed corresponding to full engine speed without shifting the transmission to the neutral position. It is desirable, in such circumstances, to be able to precisely move or "inch" the vehicle in either the forward or reverse direction in order to achieve accurate positioning and handling of materials.
Various controls and regulating valves for performing the above-stated function in conjunction with power-shift transmission employing fluid pressure actuated transmission clutches have been disclosed previously as exemplified by U.S. Pat. No. 3,143,127, to Frost, issued Aug. 4, 1964.
In order to carry out its regulating valve function, the valve means, such as described in the previously-identified patent, have been provided to effect improved gradual engagement and disengagement of the vehicle clutch after relief of excess pressures in the system by operating such valve means in conjunction with the application and release of the vehicle brakes. In such structures, wherein the operation of the valve means may be either hydraulic or mechanical, when the brakes are actuated, the valve means shifts to a position partially interrupting fluid flow from a clutch control valve and effects controlled partial deactuation of a clutch means by maintaining a relatively low fluid pressure, thus providing control of the torque to the wheels of the vehicle. In this prior art type of regulating valve control system, only one pressure, namely, the clutch pressure, has to be regulated. However, in hydrostatic transmissions, which have no clutches, control of the torque to the wheels of the vehicle must be controlled in a different manner.